An Anechoic Chamber for SPCR

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A decision was made to increase the thickness of the upper portion of this wall to match the bottom portion. The logic was simple: If blocking the window with 1.2" thick MDF board made such a big difference, perhaps making the whole wall thicker would help even more. The photos below show how this was done:

Framing was created for the new skin to be screwed into place. I used 1.25" thick pieces of wood, as well as 2x4 and 2x6 pieces for an additional center stud.

A sheet of 3/4" MDF was cut into four pieces and affixed to the framing. Rob's schedule kept him too busy, and I could not find other help; cutting up the heavy MDF sheet was the only way to handle it alone. Again, weather-stripping gaskets were used around the seams to ensure a good acoustic seal. 48'x16"x8" thick UltraTouch batting filled the cavity between the new MDF skin and the original wall / window.

After the MDF sheets were applied. The room's acoustics had changed considerably, so with the new microphone and audio system, I made a recording of a short monologue as an interim report.

Here's an acoustic interim report as a MP3 file in the room at this stage. It's easy to hear the room acoustics in this recording, especially over good headphones. It's a 2.7mb file. Caution: There are some loud handclaps, so don't set the volume too high!

The isolation from external noise was further improved, but no measurements were done to quantify the changes. Because the improvement was dramatic, and so much work was still needed for the room-in-a-room, it was too tempting to just install the echo-eliminating damping materials without any further soundproofing work.

Quotes from suppliers for acoustic foam damping wedges were discouraging. The lowest cost for wedges cut to fit the room perfectly for a 300 Hz cutoff was over $20,000, well over the funds earmarked for the project. Smaller wedge for a 400 Hz cutoff still cost $11,000, which was the entire budget. This was not a viable option; a cheaper alternative had to be found.

Damping materials are shaped into wedges for anehoic chambers because they maximize the surface area of absorption, and provide progressively increased density as sound waves approach the wall to ensure a good impedance transition. One of the objects is to create even absorption of sound at all frequencies. Discussions with several acoustics pros who had hands-on experience with anechoic chambers, including their construction, suggested that volume and mass are more important if linear frequency response is not a goal.

Chris N. Strahm, the principal engineer behind LinearX Systems, who created the industry standard loudspeaker development tool called LEAP and many other computer-based acoustics tools, pointed to photos and descriptions of his company's anechoic chamber. This anechoic chamber does not use the traditional wedges. It is fully lined with 2' cube modules made from staggered layers 3" thick fiberglass panels. A primary objective was to maximize the sheer volume and weight of damping in the chamber.

Photo copyright LinearX Systems. "A rectangular wedge design was chosen for ease of construction and yields virtually 100% material utilization. The modules were then covered with fireproof stage lining material in addition to the sprinkled room. Over 1,000 lbs of fiberglass cover the walls, ceiling, and floor of the chamber."

The results of this design are impressive and well documented on the linked pages, but the prospect of working with fiberglass was daunting. Anyone who's worked with fiberglass can tell you that it's nasty stuff. Microscopic glass fibers get everywhere, into your clothes, skin, eyes, nose, and lungs. It can be painfully irritating, and long term health effects could include cancer, though this is still unproven. I wasn't about to play with any of the stuff, never mind a thousand pounds.

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